1. Field of the Invention
The present invention relates to an ink jet type recording apparatus and more particularly to such recording apparatus comprising two recording liquid containing tanks having different capacities and a recording head mounted on a carriage. According to the present invention, the smaller capacity one of the two tanks and the recording head are united together into a single unit which is mounted on the carriage. In response to the running of the carriage, the recording liquid is automatically supplied to the smaller tank from the larger one.
2. Description of the Prior Art
The basic form of conventional ink jet recording apparatus having an open ink feed system is schematically shown in FIG. 1. The recording head 1 is composed of, for example, a piezo-electric element in a manner known per se and has a supply line 2 connected to one end part of the head. Recording liquid(ink) is fed to the recording head 1 from a tank 3 through the supply line 2. To maintain the atmospheric pressure within the tank 3 there is provided a vent hole 5 in the upper wall of the tank. The recording liquid 4 is always allowed to reach the tip end of the head 1. In response to electric signals applied to the piezo-electric element of the head 1, droplets of recording liquid are jetted from the tip end of the head 1, that is, the jet orifice 1A to effect printing of characters, marks etc. on a recording paper. The recording liquid consumed as jet droplets 6 at the recording head 1 is successively supplied from the tank 3 through the supply line 2 owing to the surface tension at the orifice 1A and the difference in liquid level between the liquid level in the tank 3 and that in the head 1. Therefore, the recording head 1 can always retain a sufficient amount of recording liquid at its orifice 1A.
The known ink jet type of recording apparatus described above involves the following problems:
One of the problems concerns the arrangement of the tank 3 and the recording head 1. When the above mentioned type of known recording apparatus is incorporated into a portable table computer or a portable typewriter, there is the possibility that the body of the apparatus may be inclined while being carried in one's hand. In this case, since the tank 3 and the tip end of the recording head 1 are arranged distant from each other, the difference between the liquid level in tank 3 and that in jet orifice 1A may be deviated from the proper value. Such a change of the liquid level difference, if occurred, will lead to retrogradation of the meniscus of recording liquid or leaking of the recording liquid from the orifice 1A. For ink jet printing it is essential to keep the meniscus formed by the jet orifice 1A at a right position. If the meniscus is moved backward into the supply line 2, the operator has to restore it to the right position. This may be done, for example, by applying a pressure to the recording liquid from the side of the tank 3. However, leakage of the recording liquid brings about some unrecoverable trouble. The leakage recording liquid will make the inner part of the apparatus dirty. In any case, these unfavorable phenomenons give the operator much trouble. Every time after transportation of the table computer or portable typewriter, the operator must do the work of restoring the retrograded meniscus or worry about any leak of recording liquid which may make the apparatus dirty.
The above mentioned unfavorable phenomenons of leaking of recording liquid or retrogradation of the meniscus will be enhanced when vibration or impact is applied to the apparatus or when the recording head 1 is struck against another member at the end of every printing at a high speed. As is well known to those skilled in the art, the meniscus at the jet orifice 1A is very sensitive to vibration or impact. If some vibrating force or impact force is applied to the body of apparatus, the recording head 1 or the supply line 2, then the meniscus is easily broken away which may result in inward retrogradation of the meniscus into the supply line 2 or leaking of droplets outward. In this case, if the difference in liquid level between tank 3 and head 1 is higher than the proper value, the meniscus once broken can not be restored to its original right position at once. The recording liquid continues to flow out from the orifice or the meniscus continues to move backward into the supply line 2 up to the position in which the liquid level difference and the surface tension get balanced finally. These vibrations and impacts are inevitable for such type of apparatus in which printing is carried out by reciprocating a recording head 1 relative to a recording medium such as printing paper. Therefore, it may be said that such sensitiveness of recording head 1 to vibration and/or impact constitutes a fatal drawback of the apparatus. For this reason, the reciprocating speed of recording head 1 is limited very much, which constitutes an obstacle against the speed-up of printing with this type of recording apparatus.
Another problem concerns air bubbles occasionally introduced into the supply line 2. Occasionally a bubble enter the supply line 2. The bubble will not particularly hinder liquid droplets from jetting from the orifice of head 1 so long as it remains in the supply line 2. However, when the bubble moves toward the head 1 accompanied by the recording liquid and enters the head, there occurs a serious problem. The bubble prevents liquid droplets from smoothly jetting out from the orifice. This trouble becomes much more serious in particular when the deformation effect of an electric-mechanical converter is used as the jet driving source of the recording head 1. In this case, energy produced by the deformation is absorbed and lost by the bubble and no energy can be transmitted to the recording liquid. Thus, the jet of the recording liquid is completely stopped and therefore a continuous and stable printing is no longer assured.
FIGS. 2A and B schematically show an example of an ink jet type recording apparatus which has been proposed to solve the problems described above. Designated by 10 is a recording head and 11 is tank. The head 10 and tank 11 are united together into a unitary component encased in a container 12. The container 12 is fixedly mounted on a carriage 13 which is in turn slidably mounted on a shaft 14. To effect printing, the carriage 13 moves along the width of a printing paper not shown. The recording head 10 is composed of a piezo-electric element 15, nozzle part 16, jet orifice 17 and supply pipe 18. The supply pipe 18 is L shaped and extends from the main body of the head 10 into the tank 11 containing recording liquid 19. The recording head 10 receives the recording liquid 19 through the supply pipe 18. To prevent the supply pipe 18 from being moved by vibration or impact, it is fixed to a wall 20 so provided as to cover the container 12. The tank 11 has a vent hole 21 to maintain the pressure within the tank 11 at atmospheric pressure. Designated by 22 and 23 are connectors for externally applying electric signals to the piezo-electric element 15 of the head 10. While not shown, the piezo-electric element 15 and the connectors 22, 23 are connected by signal lines. The orifice 17 is provided at the tip end of the nozzle part 16 and the supply pipe 18 terminates at 18A. In the shown example, the distance between 17 and 18A can be adjustably preset to a most appropriate value.
As will be seen from the foregoing, the arrangement of the apparatus shown in FIG. 2 is featured in that the recording head 10 and tank 11 are united together into a unitary member encased in a container 12 and also in that the supply pipe 18 for feeding the recording liquid to the head 10 is introduced into the tank 11 with the length of the pipe being preset to a most appropriate value. Owing to these features, the previously mentioned troubles of leakage of recording liquid from the head and retrogradation of the meniscus formed at the orifice into the supply pipe can be eliminated even when the apparatus is subjected to inclination, vibration or impact.
While the improved apparatus shown in FIG. 2 has appreciable advantages over the conventional ones, it has been found that the ink jet recording apparatus shown in FIG. 2 still involves some problems as hereinafter described.
In the case of a miniature computer or other instruments for which high speed printing is required, it is desirable to lessen the weight of the reciprocating carriage part as much as possible in view of the power of driving motor useful for driving the carriage. To meet the requirement, the amount of recording liquid to be stored in the liquid tank of the apparatus must be limited in term of weight. As an example, in case of such printer with which one character is composed of 5.times.7 dot matrix, it has been proved by experiments that when the recording head has a jet orifice of 50 to 100 .mu.m in inner diameter, only 1 cc of recording liquid is sufficient enough to print 150 to 200 thousand characters. This means that if the ink jet recording apparatus is provided with a task capacity of about 3 cc, then there can be obtained an electronic machine equipped with an ink jet recording apparatus such as a table computer with printer which is useful for a long time without any need of supply or exchange of the recording liquid. It may be possible to design such a recording liquid containing tank for which exchange of tank or supply of recording liquid is required only once every half year. However, this exchange of tank or supply of liquid to the tank brings forth a problem no matter how small the frequency of tank exchange or ink supply may be. For a table computer or other similar electronic devices there may be caused some operational troubles by the work necessary for exchange of the tank or supply of recording liquid even when the work is simplified to the utmost extent.
Another problem is caused by change of weight load on the carriage carrying the tank. As the recording liquid in the tank is consumed, the weight load on the carriage changes gradually with time. Assuming that there is used a tank having 3 cc capacity, the change of tank weight will reach about 3 g when comparison is made between the weight of the tank being full and that being vacant. When a linear motor or the like is used as the carriage driving motor, this change of weight loaded on the carriage will cause a change of carriage driving speed and also a change of printing speed. Since the printing speed varies from time to time, it is no longer possible to keep the print quality at a desired level.